The present invention relates to a combustion-type power tool, and more particularly, to a combustion-type fastener driving tool in which combustible gas is ejected from a gas canister into a combustion chamber, mixed with air and ignited to drive a piston, thus generating power to drive nails or the like.
FIG. 3 illustrates a conventional combustion-type driving tool 101. The tool 101 generally includes a housing 102, a head cap 104, a combustion-chamber frame 106, a push lever (not shown), a cylinder 103, a piston 109, a driver blade (not shown), a motor 112, a fan 113, a gas canister (not shown), an ignition plug body 116, an exhaust-gas check valve (not shown), a magazine (not shown), and a tail cover (not shown). The head cap 104 is positioned in the housing 102. The combustion-chamber frame 106 is movable in the housing 102 in the lengthwise direction thereof. One end of the frame 106 is abutable on the head cap 104. The push lever is movably provided at the other end of the housing and is coupled to the combustion-chamber frame 106. The cylinder 103 is secured to the housing 102 in communication with the combustion-chamber frame 106. The cylinder 103 guides the movement of the combustion-chamber frame 106 and is formed with an exhaust port. The piston 109 is reciprocally movable in the cylinder 103. While the combustion-chamber frame 106 has its one end abutting on the head cap 104, the piston 109 defines a combustion chamber 111 in cooperation with the head cap 104, the combustion-chamber frame 106 and the end portion of the cylinder 103, the end portion being positioned near the head cap. The driver blade extends from the end of the piston 109 which faces away from the combustion chamber 111 toward the other end of the housing 102. The motor 112 is supported on the head cap 104. The fan 113 is fastened to the motor 112 and provided in the combustion chamber 111. The gas canister is assembleable in the housing 102 and contains combustible gas that is to be ejected into the combustion chamber 111 through a gas passage formed in the head cap 104. The ignition plug body 116 is faced to the combustion chamber 111 to ignite a mixture of combustible gas and air. The exhaust-gas check valve selectively closes the exhaust port. The magazine is positioned at the other end of the housing 102 and contains fastening elements such as nails. The tail cover is interposed between the magazine and the push lever to supply the fastener from the magazine to a position of a moving locus of the driver bit.
The combustible gas is ejected into the combustion chamber 111 from the gas canister assembled in the housing 102. In the combustion chamber 111, the combustible gas and air are stirred and mixed together by the fan 113. The ignition plug body 116 ignites the resultant mixture gas. The mixture gas explodes to drive piston 109 for driving the driver blade, which in turn drives nails into a workpiece such as a wood block. Such conventional power tool is disclosed in U.S. Pat. No. 5,197,646 and Japanese Patent Publication No. Hei 3-25307.
The combustion-type driving tools disclosed in these publications provide a head cap 104 having a structure as shown in FIG. 4. In the combustion-type driving tool of FIG. 4, the ignition space is positioned in a displaced position, while communicating with the main combustion space. Thus, static state of the gas mixture can be obtained in the main combustion space by restraining wind pressure due to a flow of the gas mixture, in order to prevent the ignited flame from being puffed out. More specifically, the head cap 104 surface part defining a part of the combustion chamber includes a first part 104A, a second part 104B and a third part 104C. The first part 104A lies around an axis 113a of the fan 113. The second part 104B supports the ignition plug body 116. The third part 104C lies at an outer side of the second part 104B in the radial direction of the fan 113. An electrode 116A of the plug 116 is positioned at the second part 104B of the head cap 104. The second part 104B is located farther from the piston 109 than the first and third parts 104A, 104C to the piston 109. The second part 104B defines a projecting space 104a functioning as an ignition space.
The electrode 116A of the ignition plug body 116 and an opposing electrode section 156 oppose to each other. The electrode section 156 has a protruding section protruding around the axis 113a and in a circumferential direction of the fan 113. By virtue of this positional relation between the electrode 116A and the opposing electrode section 156, a main stream A and a sub-stream B develop in the combustion chamber 111 as shown in FIG. 3, when the fan 113 is rotated and stirs and mixes the combustible gas with air in a sealed state of the combustion chamber 111. As a result, the sub-stream B passes through a gap between the electrode 116A and the opposing electrode section 156 as illustrated in FIG. 4. The sub-stream B much hinders the ignition of the combustible gas. The sub-stream B becomes particularly intensified when the rotation speed of the fan 113 is increased in order to improve output or scavenging efficiency. In the latter case, ignition to the gas-fuel mixture becomes more difficult.
A cover may be provided to cover both the electrode 116A and the opposing electrode section 156 to improve ignitability of the gas-fuel mixture. However, this not only increases the number of components of the combustion-type driving tool to increase production cost, but also makes it troublesome to clean the interior of the combustion chamber. Thus, maintenance to the combustion chamber becomes degraded.